Device and method for measuring element temperature of air-fuel ratio sensor, and device and method for controlling heater of air-fuel ratio sensor

ABSTRACT

A device according to the present invention is provided with element temperature measurement voltage application circuit for temporarily applying a predetermined voltage for element temperature measurement to a sensor element of an air-fuel ratio sensor equipped to an exhaust system of an internal combustion engine, first sensor output reading circuit for reading in a sensor output just before applied with the voltage, and second sensor output reading circuit for reading in a sensor output being applied with the voltage, wherein the element temperature of the air-fuel ratio sensor is estimated based on the sensor output just before applied with the voltage and the sensor output being applied with the voltage. Further, during internal resistance measurement of the sensor element of the air-fuel ratio sensor equipped to the exhaust system of the internal combustion engine, a voltage applied to the heater for heating the sensor element is maintained to be constant.

FIELD OF THE INVENTION

The present invention relates to a technique for measuring the elementtemperature of an air-fuel ratio sensor (including an oxygen sensor)equipped to an exhaust system of an internal combustion engine andutilized for controlling an air-fuel ratio of the engine, and to atechnique for controlling a heater for heating a sensor element equippedto the air-fuel ratio sensor based on the measured element temperature.

DESCRIPTION OF THE RELATED ART

Heretofore, there has been known an air-fuel ratio control device of aninternal combustion engine for detecting an actual air-fuel ratio of theengine based on the oxygen concentration and the like in the exhaustusing an air-fuel ratio sensor, and performing a feedback control offuel supply quantity to the engine so that the actual air-fuel ratioreaches a target air-fuel ratio.

In order to perform the above-mentioned air-fuel ratio feedback control,it is required that the air-fuel ratio sensor is already activated.Since the air-fuel ratio sensor is activated when the elementtemperature reaches a predetermined activation temperature, the air-fuelratio sensor is equipped with a heater for heating the sensor element,so as to control the element temperature to the target temperature bycontrolling the power supply to the heater.

Specifically, the internal resistance of the sensor element is measured,and based on the element temperature estimated from the measuredresistance, a power supply amount to the heater is feedback controlledso that the element temperature reaches the target temperature (refer toJapanese Unexamined Patent Publication Nos. 8-278279,61-122556,11-344466, etc.).

However, in a case where, for measuring the element temperature of theair-fuel ratio sensor (or the internal resistance of the sensor elementrelated thereto), a predetermined voltage for measuring the elementtemperature (or for measuring the internal resistance) is applied to thesensor element, to measure the internal resistance based on the sensoroutput at that time, a voltage for detecting the air-fuel ratio iscontinuously output from the air-fuel ratio sensor even during internalresistance measurement (during application of measurement voltage).Therefore, if the sensor output is used as it is for measurement ofinternal resistance, an estimation error of the element temperaturebecomes too large.

Moreover, in recent years, the sensor element has been miniaturized andthe capacity of the heater has been increased for the purposes ofactivating the sensor element quickly and maintaining the activatedstate securely, thereby improving the temperature follow-up capabilityof the sensor element against the power supply to the heater (relativelyreducing the heat capacity of the sensor element). Therefore, especiallyin an air-fuel ratio control device wherein the power supply to theheater is controlled by performing a duty-control of the ON/OFF of theheater power supply, due to this ON/OFF of the heater power supply, thesensor element temperature is fluctuated momentarily and the internalresistance of the element is also fluctuated. This causes an error inthe measurement of internal resistance, to lead to the deterioration ofestimation accuracy of the element temperature.

Such deterioration of estimation accuracy of the element temperature notonly causes the deterioration of the ability to control the element tothe target temperature when controlling the power supply to the heaterfor heating the sensor element, but also causes the increase of thepower consumption by the heater. It further causes a bad influence tothe feedback control of the air-fuel ratio.

SUMMARY OF THE INVENTION

The present invention has been achieved in view of the aboveconventional problems and has an object to enable a more accuratemeasurement of the element temperature in an air-fuel ratio sensor.

Another object of the present invention is to enable an accuratejudgment of the activation status of the air-fuel ratio sensor based onthe accurately measured element temperature.

Yet another object of the present invention is to control a sensorelement to the desired element temperature by controlling a heater forheating the sensor element based on the accurately measured elementtemperature.

Still another object of the present invention is to improve the accuracyof an air-fuel ratio feedback control based on the above heater control.

In order to achieve the above objects, the present invention isconstituted as follows.

When applying a predetermined voltage for measuring the elementtemperature to the sensor element of the air-fuel ratio sensor, both asensor output just before applied with the predetermined voltage as wellas a sensor output being applied with the predetermined voltage are readin.

Based on both the sensor output just before applied with thepredetermined voltage and the sensor output being applied with thevoltage, the sensor element temperature is estimated.

With this constitution, since there can be considered an influence ofvoltage output for air-fuel ratio detection of the air-fuel ratio sensorby the sensor output just before applied with the voltage, theestimation accuracy of the sensor element temperature is improved,thereby enabling an accurate activation judgment and the like of theair-fuel ratio sensor.

Using the sensor output being applied with the predetermined voltage asa basis, and using the sensor output just before applied with thevoltage as a correction parameter, the element temperature can beestimated, and further, by computing the internal resistance of thesensor element based on the sensor output, the element temperature canbe estimated.

Even further, during internal resistance measurement of the sensorelement of the air-fuel ratio sensor, a voltage applied to the heaterfor heating the sensor element is maintained to be constant.

Thereby, it becomes possible to prevent the fluctuation of sensorelement temperature (in other words, the fluctuation of internalresistance of the sensor element), which is caused by the fluctuation ofheater application voltage accompanying the control of the heater forheating the sensor element, so that the internal resistance of thesensor element can be measured accurately, and the element temperaturecan be accurately computed based on the measured internal resistance.

Further, it becomes possible to feedback control the heater for heatingthe sensor element, based on the accurately detected elementtemperature, so that the element temperature reaches a targettemperature, thereby enabling the sensor element to be controlled to thedesired temperature.

The other objects and features of the present invention will becomeunderstood from the following description with reference to accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system diagram of an air-fuel ratio feedback control deviceof an engine showing an embodiment of the present invention;

FIG. 2 is a diagram showing the structure of sensor element of theair-fuel ratio sensor;

FIGS. 3(a) and 3(b) is a characteristic chart showing the sensor elementof the air-fuel ratio sensor;

FIG. 4 is a first control circuit diagram for a heater and the sensorelement of the air-fuel ratio sensor;

FIG. 5 is a flowchart showing the element temperature measurementroutine (a first embodiment);

FIG. 6 is a time chart of the element temperature measurement;

FIG. 7 is a flowchart showing the heater control routine;

FIG. 8 is a flowchart showing the element temperature measurementroutine according to a second embodiment;

FIG. 9 is a flowchart showing the element temperature measurementroutine according to a third embodiment;

FIG. 10 is a second control circuit diagram for the heater and thesensor element of the air-fuel ratio sensor;

FIG. 11 is a flowchart showing the element temperature measurementroutine according to a fourth embodiment;

FIGS. 12(a) and 12(b) is a diagram showing the duty control (ON/OFFstate) during impedance measurement;

FIGS. 13(a) and 13(b) shows another control circuit diagram for theheater of the air-fuel ratio sensor; and

FIG. 14 is a flowchart showing the element temperature measurementroutine according to a fifth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will now be explained.

FIG. 1 is a system diagram showing an air-fuel ratio feedback controldevice of an internal combustion engine.

The internal combustion engine (hereinafter referred to as engine) 1 isprovided with, for each cylinder, a fuel injection valve 3 facing eitheran intake passage 2 or a combustion chamber. A control unit 4 controlsthe fuel injection performed by each fuel injection valve 3.

The control unit 4 computes a basic fuel injection quantity Tp=K×Qa/Ne(wherein K is constant) equivalent to stoichiometric amount of air(λ=1), from for example an intake air quantity Qa detected based on asignal output from an airflow meter 5 and an engine rotation speed Nedetected based on a signal output from a crank angle sensor 6. Thecomputed basic fuel injection quantity is then corrected by both atarget air-fuel ratio tλ and an air-fuel ratio feedback correctioncoefficient < based on a signal output from an air-fuel ratio sensor 8disposed in an exhaust passage 7, thereby computing a final fuelinjection quantity Ti=Tp×(1/tλ)×α. A fuel injection pulse having a pulsewidth corresponding to the computed fuel injection quantity Ti is outputto each fuel injection valve 3 in synchronism with the engine rotation.

Here, the air-fuel ratio sensor 8 is disposed in the exhaust passage 7for outputting signals corresponding to the oxygen concentration in theexhaust. Based on the signals output from the air-fuel ratio sensor 8,the control unit 4 detects an air-fuel ratio λ of the air-fuel mixturesupplied to the engine 1, and increasingly/decreasingly sets theair-fuel ratio feedback correction coefficient α using aproportional-plus-integral control and the like, to feedback control theair-fuel ratio λ of the air-fuel mixture so that the air-fuel ratio λ ofthe air-fuel mixture reaches the target air-fuel ratio tλ.

Moreover, the air-fuel ratio sensor 8 is a so-called wide-range air-fuelratio sensor capable of detecting an air-fuel ratio linearly by varyingan output voltage thereof continuously in accordance with the air-fuelratio, which is equipped with a heater for heating a sensor element.

The structure of the air-fuel ratio sensor 8 is shown in FIG. 2.

In FIG. 2, a sensor element body 20 is formed into porous layers withsolid electrolyte material such as zirconia having oxygen ionconductivity. Within the sensor element body 20, there is provided aheater 21, an atmosphere chamber 22, and a gas diffusion chamber 23,from the bottom up in the figure.

The heater 21 heats a sensor element when power is supplied to theheater 21.

The atmosphere chamber 22 is formed so as to communicate with theatmosphere being the reference gas, outside the exhaust passage.

The gas diffusion chamber 23 is formed to communicate with the exhaustthrough an exhaust introduction hole 24 formed to the upper surface sideof the body 20 in the figure, and through a protection layer 25 formedof γ alumina and the like.

A Nernst cell portion 26 is constituted by an electrode 26A formed onthe upper wall of the atmosphere chamber 22 and an electrode 26B formedon the lower wall of the gas diffusion chamber 23.

Moreover, a pump cell portion 27 is constituted by an electrode 27Aformed on the upper wall of the gas diffusion chamber 23 and anelectrode 27B formed on the upper wall of the body 20 and covered by theprotection layer 28.

The Nernst cell portion 26 generates a voltage in accordance with anoxygen partial pressure between the Nernst cell portion electrodes 26Aand 26B influenced by the oxygen ion concentration (oxygen partialpressure) within the gas diffusion chamber 23. Therefore, by detectingthis voltage, it is possible to detect whether the air-fuel ratio isrich or lean relative to the stoichiometric amount of air (λ=1).

When a predetermined voltage is applied to the pump cell portion 27, theoxygen ion within the gas diffusion chamber 23 moves so that the currentflows between the pump cell portion electrodes 27A and 27B. Since thecurrent value (limit current value) Ip flowing between the pump cellportion electrodes 27A and 27B when a predetermined voltage is appliedthereto is influenced by the oxygen ion concentration within the gasdiffusion chamber 23, by detecting the current value Ip, the air-fuelratio of the exhaust can be detected.

In other words, as shown in FIG. 3A, since a voltage-currentcharacteristic of the pump cell portion 27 is varied in accordance withthe air-fuel ratio λ, the air-fuel ratio of the exhaust can be detectedfrom the current value Ip when a predetermined voltage Vp is applied tothe pump cell portion 27.

Moreover, based on the lean/rich output by the Nernst cell portion 26,the direction of voltage application to the pump cell portion 27 isreversed, so that in both the lean and rich regions of the air-fuelratio, a wide range detection of air-fuel ratio λ can be performed basedon the current value Ip flowing through the pump cell portion 27, asshown in FIG. 3B.

FIG. 4 shows a first control circuit for the sensor element 20 andheater 21 for heating the sensor element in the air-fuel ratio sensor 8.

An output voltage Vs of the sensor element 20 in the air-fuel ratiosensor 8 is varied continuously in accordance with the air-fuel ratio,and this output Vs is input to the control unit 4.

Further, a predetermined voltage Vcc (for example, 5V) for measuring theelement temperature (for measuring the internal resistance) of thesensor element is applied to the sensor element 20 via a switchingelement 13 and a reference resistance R0.

Therefore, if the switching element 13 is turned ON during elementtemperature measurement, the voltage for measuring the elementtemperature is superimposed on the output Vs of the sensor element 11.

A battery voltage VB is applied to the heater 21, and a switchingelement 14 is disposed in a power supply circuit.

A CPU 15 within the control unit 4 reads the output Vs of the sensorelement 20 via a filter (smoothing circuit) 16 and an A/D converter 17,at a predetermined timing, while controlling the ON/OFF of the switchingelement 13 for applying voltage used for measuring the elementtemperature.

Moreover, the CPU 15 performs a duty-control of the ON/OFF of theswitching element 14 for controlling the heater via a D/A converter 18,to thereby control the power supply amount to the heater 21.

Next, the contents of control of the CPU 15 will be explained withreference to a flowchart.

FIG. 5 is a flowchart showing the routine for measuring the elementtemperature according to a first embodiment, to be executed at apredetermined crank angle cycle.

In step 1 (abbreviated as “S1” in the drawing, the same holdshereinafter), the sensor output Vs is read in and set to Vaf=Vs, basedon which the air-fuel ratio λ is detected.

In step 2, the switching element 13 is turned ON, and the application ofvoltage Vcc for measuring the element temperature to the sensor element20 is started. In other words, immediately after reading the sensoroutput for air-fuel ratio detection, the application of voltage Vcc formeasuring the element temperature is started.

In step 3, after a first predetermined time T1 has passed from thestarting of application of voltage for measuring the elementtemperature, the sensor output Vs is read in and set to Vr=Vs so as tomeasure the internal resistance of the sensor element 20.

In step 4, the sensor output Vr being applied with voltage is correctedby the sensor output Vaf just before applied with voltage. Specifically,the sensor output Vaf just before applied with voltage is subtractedfrom the sensor output Vr being applied with voltage, to thereby obtaina corrected sensor output Vr=Vr−Vaf.

In step 5, the internal resistance Rs of the sensor element 20 iscomputed based on the corrected sensor output Vr.

Specifically, when the current flowing through sensor element 20 is i,and Vs=Vr,

Vr=i×Rs

Vcc−Vr=i×R 0.

Therefore, based on the above equations, the internal resistance Rs canbe computed by

Rs=Vr/[(Vcc−Vr)/R 0]

In step 6, based on the internal resistance Rs of the sensor element 20,the element temperature Ts is computed for example by referring to atable. As the element temperature Ts becomes higher, the internalresistance Rs decreases, so the element temperature Ts can be computedfrom the internal resistance Rs.

In step 7, the switching element 13 is turned OFF after a secondpredetermined time T2 has passed from the starting of application ofvoltage for measuring the element temperature, to thereby stop(terminate) the application of voltage Vcc for measuring the elementtemperature to the sensor element 11.

Effects of the above-mentioned element temperature measurement will beexplained with reference to FIG. 6.

In a case where the element temperature measurement voltage is appliedto the sensor element of the air-fuel ratio sensor, since the sensoroutput is superimposed with an output signal equivalent to the oxygenbattery of the sensor element, it is likely that the sensor output beingapplied with voltage is varied even if the element temperature isconstant, thereby causing an error in estimating the elementtemperature.

Therefore, by correcting the sensor output Vr being applied with voltageby the sensor output Vaf just before applied with voltage, to bespecific, by setting the corrected sensor output Vr to Vr=Vr−Vaf, aninfluence of the oxygen battery is eliminated, so that the internalresistance Rs is correctly computed based on the corrected sensor outputVr, and the estimation error of the element temperature Ts iseliminated.

Moreover, when detecting the air-fuel ratio by reading the sensor outputat a predetermined crank angle cycle, the voltage for measuring theelement temperature is applied to the sensor element immediately afterreading the sensor output for air-fuel ratio detection. This enables tominimize the number of times in which, since the voltage is beingapplied, the air-fuel ratio is unable to be detected at air-fuel ratiodetection timings during high speed rotation and the like, and itfurther enables to minimize the influence to the air-fuel ratio controlperformance. Also, the sensor output for air-fuel ratio detection can beread instead of sensor output just before applied with voltage.

FIG. 7 is a flowchart of the heater control routine, to be executed ateach predetermined time.

In step 101, the latest element temperature Ts computed by the routineof FIG. 5 is read in.

In step 102, in accordance with a deviation between the actual elementtemperature Ts and the target temperature, a heater duty HDUTY (%) iscomputed using a known PID control, so as to approximate the elementtemperature Ts to the target temperature.

To be specific, when the actual element temperature Ts is lower than thetarget temperature, the heater duty HDUTY is increased so as to increasethe power supply amount (power supply time ratio) to the heater 21. Incontrast, when the actual element temperature Ts is higher than thetarget temperature, the heater duty HDUTY is decreased so as to reducethe power supply amount (power supply time ratio) to the heater 21.

In step 103, the computed heater duty HDUTY is output, whereby theswitching element 14 is switched ON or OFF to control the power supplyamount to the heater 12, thereby converging the element temperature Tsto the target temperature.

Incidentally, in the above embodiment, the internal resistance Rs of thesensor element 20 is measured, based on which the element temperature Tsis computed, thereby feedback controlling the element temperature Ts toreach the target temperature during heater control. However, since theelement temperature Ts is determined based on the internal resistanceRs, the internal resistance Rs may be feedback controlled to reach thetarget internal resistance during heater control without computing theelement temperature Ts.

In this case, when the actual internal resistance Rs is greater than thetarget internal resistance, the element temperature is low, so theheater duty HDUTY is increased so as to increase the power supply amountto the heater 21. In contrast, when the actual internal resistance Rs issmaller than the target internal resistance, the element temperature ishigh, so the heater duty HDUTY is decreased so as to reduce the powersupply amount to the heater 21.

Next, other embodiments according to the present invention will beexplained.

FIG. 8 is a flowchart of the element temperature measurement routineaccording to a second embodiment, to be executed instead of theflowchart of FIG. 5.

Steps 51 through 53 are the same as steps 1 through 3 in the flowchartof FIG. 5.

In step 54, the internal resistance Rs of the sensor element 11 iscomputed based on the sensor output Vr being applied with voltagewithout correction.

In step 55, the correction value for the internal resistance Rs (Rscorrection value) is computed from the sensor output Vaf just beforeapplied with voltage.

In step 56, the internal resistance Rs computed in step 54 is correctedby the Rs correction value computed in step 55.

The correction here is performed so that when the sensor output Vaf justbefore applied with voltage is greater, the internal resistance Rs iscorrected to a smaller value, since when the sensor output Vaf justbefore applied with voltage is greater, the internal resistance Rs iscomputed to be greater than the actual value.

Then, in step 57, the element temperature Ts is computed based on theinternal resistance Rs (corrected Rs) of the sensor element 11 byreferring to a table and the like. Step 58 is the same as step 7 in theflowchart of FIG. 5.

FIG. 9 is a flowchart showing the element temperature measurementroutine according to a third embodiment, to be executed instead of theflowchart of FIG. 5.

Steps 61 through 63 are the same as steps 1 through 3 in the flowchartof FIG. 5.

In step 64, the internal resistance Rs of the sensor element 20 iscomputed based on the sensor output Vr being applied with voltagewithout correction. In step 65, the element temperature Ts is computedbased on the internal resistance Rs of the sensor element 20 byreferring to a table and the like.

In step 66, the correction value for element temperature Ts (Tscorrection value) is computed based on the sensor output Vaf just beforeapplied with voltage.

In step 67, the element temperature Ts computed in step 64 is correctedby the Ts correction value computed in step 66.

The correction here is performed so that when the sensor output Vaf justbefore applied with voltage is greater, the internal resistance Rs iscorrected to a smaller value, since when the sensor output Vaf justbefore applied with voltage is greater, the internal resistance Rs iscomputed to be greater than the actual value and the element temperatureTs is therefore computed to be lower than the actual temperature.Accordingly, the corrected element temperature Ts (corrected Ts) is usedfor controlling the heater and the like. Step 68 is the same as step 7in the flowchart of FIG. 5.

Next, an explanation will be made for where the impedance of the sensorelement of the air-fuel ratio sensor 8 is measured, and the elementtemperature is measured based on the measured impedance.

FIG. 10 shows a second control circuit for the sensor element portion 20of the air-fuel ratio sensor (Nernst cell portion 26, pump cell portion27) and the heater 21 for heating the sensor element.

In the present control circuit, under the control of a microcomputer 30,an alternating voltage is applied from an AC power source 31 to theNernst cell portion 26 for measuring impedance, and the current value Isflowing through the Nernst cell portion 26 is voltage converted anddetected by a current detecting resister 32 and a detecting amplifier33.

A signal from the detecting amplifier 33 is input for example to animpedance detecting circuit 34 comprising a high pass filter and anintegrator, so that only an alternating current component is taken outto detect the impedance Ri from the amplitude of the alternatingcomponent. Thereby, the impedance Ri of the Nernst cell portion 26 canbe measured.

Moreover, the signal from the detecting amplifier 33 is input to a lowpass filter 35, so that only a direct current component is taken out todetect a voltage generated at the Nernst cell portion 26 correspondingto the oxygen concentration. Thereby, the lean/rich of oxygenconcentration can be detected.

Under the control of the microcomputer 30, a predetermined voltage Vp isapplied by a DC power source 36 to the pump cell portion 27, but thedirection of application is inverted corresponding to the rich/lean ofthe oxygen concentration detected by the Nernst cell portion 26, so thatthe current value Ip flowing through the pump cell portion 27 is voltageconverted and detected by a current detecting resister 37 and adetecting amplifier 38. Thereby, the air-fuel ratio λ is detected.

A battery is used to apply a battery voltage VB to the heater 21, butsince a switching element 39 is disposed in a power supply circuitsimilar to the first control circuit (FIG. 4), normally themicrocomputer 30 performs a duty-control of the ON/OFF of the switchingelement 39, thereby enabling the control of the power supply amount tothe heater 21.

FIG. 11 is a flowchart showing the element temperature measurementroutine according to a fourth embodiment, to be executed at eachpredetermined time by the microcomputer 30.

In step 71, various operating conditions are read in.

In step 72, it is judged whether or not an impedance Ri measurementpermitting condition is fulfilled. Here, the impedance Ri measurementpermitting condition is fulfilled when an influence of heat-draw (?)caused by a change in exhaust flow rate is small, for example, when theoperating condition of the engine is within a predetermined rotationspeed Ne and a predetermined fuel injection quantity Tp.

When the impedance measurement permitting condition is not fulfilled,the procedure returns to step 71.

When the impedance measurement permitting condition is fulfilled, theprocedure advances to step 73, where the heater duty (HDUTY) is set to 0(%), and after stopping the power supply to the heater 21, the procedureadvances to step 74.

In step 74, the impedance of the sensor element (Nernst cell portion 26)is measured. Specifically, a predetermined alternating voltage isapplied to the Nernst cell portion 26 from the AC power source 31, and aterminal voltage of the current detecting resister 32 at that time isread in, based on which the impedance Ri of the Nernst cell portion 26is measured.

That is, as shown in FIG. 12A, during measurement of impedance Ri, thepower supply to the heater for heating the sensor element is turned OFF(applied voltage: 0V) and this state is maintained.

Thereafter, the procedure advances to step 75, where the elementtemperature is detected based on the impedance Ri measured in step 74,by searching a table and the like set in advance of the elementtemperature and a theoretical value of impedance Ri.

After terminating the above impedance measurement, a normal duty controlis resumed by the power supply to the heater 21.

Based on the above routine, the power supply to the heater for heatingthe sensor element is stopped (maintaining the applied voltage to beconstant as 0V) during impedance measurement of the sensor element sothat the impedance can be measured at the element temperatureapproximately equal to the exhaust temperature at that time whilepreventing the momentary fluctuation of the sensor element temperature.Thereby, the impedance measurement accuracy of the sensor element isimproved, and further, the detection accuracy of the sensor elementtemperature is improved.

Moreover, in step 73 of the flowchart of FIG. 11, during impedancemeasurement of the sensor element, the heater duty (HDUTY) may be set to100 (%), to turn the power supply to the heater ON and to maintain thisstate, as shown in FIG. 12B. Therefore, the applied voltage can bemaintained to be constant as the maximum set value and the momentaryfluctuation of the sensor element temperature is prevented, to therebyenable the impedance measurement in a stable state.

The voltage during impedance measurement may be set in advance byresistance adjustment and the like without performing theabove-mentioned duty control. A control circuit for controlling theheater for the sensor element in such a case is shown in FIG. 13.

As shown in FIG. 13A, the present control circuit is constituted tocontrol the power supply amount of the heater 21 by performing theduty-control of the ON/OFF of the switching element 39 by themicrocomputer 30 in a normal state, similar to the above-mentionedembodiment, whereas during impedance measurement, to select a voltagevalue set in advance by resistance adjustment of a switch 40 forapplication to the heater 21.

The switch 40 sets an applied voltage value for the heater for exampleby arbitrarily selecting a plurality of resistances as shown in FIG.13B.

FIG. 14 is a flowchart of the element temperature measurement routineaccording to a fifth embodiment.

Steps 81 and 82 are the same as steps 71 and 72 in the flowchart of FIG.11, so explanations thereof are omitted.

When the impedance Ri measurement permitting condition is fulfilled instep 82, the procedure advances to step 83.

In step 83, the target heater voltage during impedance measurement isdetermined by searching through a map and the like set in advance basedon the read engine rotation speed Ne and fuel injection quantity Tp.Thereby, the applied voltage can be set corresponding to the exhausttemperature approximately equal to the element temperature, to preventthe fluctuation of element temperature.

Next, in step 84, the resistance of the switch 40 is adjusted so as toset the hater voltage to the target heater voltage determined in step83. Then, the procedure advances to step 85, where the impedance Ri ismeasured.

Then, the procedure advances to step 86 where the table and the like setin advance of the element temperature and the theoretical value ofimpedance Ri are searched, to detect the element temperature based onthe impedance Ri measured in step 85.

From the above, during impedance measurement, the voltage set based onthe engine rotation speed Ne and the fuel injection quantity Tp isapplied to the heater, thereby preventing the fluctuation of elementtemperature and also minimizing the variation of element temperaturebefore and after impedance measurement, thereby enabling even moreaccurate impedance measurement.

Further, the present invention is not limited to the above embodimentswhere the heater control is performed by a duty control (in a normalstate), but can achieve the same advantageous effects even in a casewhere the heater control is performed by any other control method, suchas a method of controlling the applied voltage to the heater.

The entire contents of Japanese Patent Application No. 2000-144427,filed May 17, 2000, and Japanese Patent Application No. 2000-197020,filed Jun. 29, 2000, are herein incorporated by reference.

What is claimed:
 1. A device for measuring element temperature of anair-fuel ratio sensor comprising: a heater for heating a sensor elementof said air-fuel ratio sensor equipped to an exhaust system of aninternal combustion engine; an internal resistance measurement circuitfor measuring internal resistance of said sensor element; an arithmeticcircuit for computing the element temperature based on said measuredinternal resistance, and computing a power supply amount to said heaterfor heating the sensor element based on said computed elementtemperature; a heater control circuit for controlling said heater basedon said computed power supply amount; and a heater applied voltagecontrol circuit for maintaining a heater applied voltage to said heaterfor heating the sensor element to be constant during internal resistancemeasurement of the sensor element.
 2. The device according to claim 1,wherein said air-fuel ratio sensor is equipped with a Nernst cellportion for generating an element voltage corresponding to lean/rich ofan air-fuel ratio, and a pump cell portion for being applied with apredetermined pump voltage in a direction corresponding to the lean/richof the air-fuel ratio detected by said Nernst cell portion, tocontinuously vary a current value thereof corresponding to the air-fuelratio; and wherein said internal resistance measurement circuit measuresinternal resistance of said Nernst cell portion based on a current valueflowing through said Nernst cell portion when an alternating voltage forinternal resistance measurement is applied to said Nernst cell portion.3. The device according to claim 1, wherein said heater control circuitcontrols the power supply amount to the heater by performing a dutycontrol of the ON/OFF of the power supply to the heater; and whereinsaid heater applied voltage control circuit maintains the power supplyto the heater to an OFF state during internal resistance measurement ofsaid sensor element.
 4. The device according to claim 1, wherein saidheater control circuit controls the power supply amount to the heater byperforming a duty control of ON/OFF of the power supply to the heater;and wherein said heater applied voltage control circuit maintains thepower supply to the heater to an ON state during internal resistancemeasurement of said sensor element.
 5. A device for measuring elementtemperature of an air-fuel ratio sensor comprising: a heater for heatinga sensor element of said air-fuel ratio sensor equipped to an exhaustsystem of an internal combustion engine; internal resistance measurementmeans for measuring internal resistance of said sensor element; heaterpower supply amount computing means for detecting the sensor elementtemperature based on said measured internal resistance, to compute apower supply amount to said heater for heating the sensor element basedon said detected element temperature; heater control means forcontrolling said heater based on said computed heater power supplyamount; and heater applied voltage control means for maintaining aheater applied voltage to said heater for heating the sensor element tobe constant during internal resistance measurement of the sensorelement.
 6. A method for measuring element temperature of an air-fuelratio sensor comprising the steps of: measuring internal resistance of asensor element of said air-fuel ratio sensor equipped to an exhaustsystem of an internal combustion engine, computing the elementtemperature based on said measured internal resistance, and computing apower supply amount to a heater for heating the sensor element based onsaid computed element temperature; and maintaining a heater appliedvoltage to said heater for heating the sensor element to be constantduring internal resistance measurement of the sensor element.
 7. Themethod according to claim 6, wherein said air-fuel ratio sensor isequipped with a Nernst cell portion for generating an element voltagecorresponding to lean/rich of an air-fuel ratio, and a pump cell portionfor being applied with a predetermined pump voltage in a directioncorresponding to the lean/rich of the air-fuel ratio detected by saidNernst cell portion, to continuously vary a current value thereofcorresponding to the air-fuel ratio; and wherein internal resistance ofsaid Nernst cell portion is measured based on a current value flowingthrough said Nernst cell portion when an alternating voltage forinternal resistance measurement is applied to said Nernst cell portion.8. The method according to claim 6, wherein the power supply amount tothe heater for heating the sensor element is controlled by performing aduty control of ON/OFF of the power supply to the heater; and whereinthe power supply to the heater is maintained to an OFF state duringinternal resistance measurement of said sensor element.
 9. The methodaccording to claim 6, wherein the power supply amount to the heater forheating the sensor element is controlled by performing a duty control ofON/OFF of the power supply to the heater; and wherein the power supplyto the heater is maintained to an ON state during internal resistancemeasurement of said sensor element.